An inverter controlled generator set comprises an engine, a DC power source section having as a power source a generator driven by the engine and an inverter converting an output of the DC power source section into an AC output to supply to a load as disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 2001-128463.
Generally, an AC generator is used as a generator driven by an engine. In this case, a DC power source section comprises the generator, a rectifier circuit rectifying an AC output of the generator and a capacitor smoothing an output of the rectifier circuit.
The inverter comprises: a bridge type inverter circuit, each branch of a bridge being constituted by switch elements such as a transistor or a FET, and into which an output of the DC power source section is input; a controller for controlling the switch elements that constitute the inverter circuit so as to output an AC voltage having a predetermined frequency and a predetermined peak value from the inverter circuit; and a filter circuit which removes a high harmonic component from the AC voltage output from the inverter circuit.
The controller comprises a microprocessor and controls the switch elements constituting the inverter circuit so as to output from inverter a rated AC voltage having a constant effective value and a constant frequency.
In order to output the AC voltage having a desired rated value from the inverter controlled generator set of this type, it is necessary that an output voltage (an average value of the output voltage of the generator) of the DC power source section is higher than a voltage having a value which is a sum of a peak value of the output voltage (the AC voltage) of the inverter, a voltage drop generated in the switch elements of the inverter circuit and a voltage drop generated in the filter circuit. For example, in the case where a rated output voltage an effective value of which is 100V is generated from the inverter, it is necessary for the DC power source section to generate a DC voltage higher than an added voltage (150˜160V) of a voltage (10˜20V), which is the sum of the voltage drop generated in the switch elements of the inverter circuit and the voltage drop generated in the filter circuit, and approximately 140V of a peak value of the rated output voltage.
In general, when the engine is started, the rotational speed is gradually increased, and the output voltage of the generator is increased in accordance with the increase of the rotational speed. In a process of increasing the output voltage of the generator, a microprocessor of a controller is started first. Although the inverter becomes controllable if the microprocessor is started, the output voltage of the DC power source section has not reach a value required for outputting a rated voltage from the inverter when the microprocessor is started. Thus, if the control of the inverter is started as soon as the microcomputer is stared, the inverter generates only a lower voltage than the rated voltage.
If a load is connected to the inverter when the inverter is started under a condition that the rotational speed and the output voltage of the DC power source section (generator) is insufficient, the output voltage of the generator may become lower, and the load may not be started. Also, in the state where a warm-up of the engine is not completed, the engine is in an unstable state. Therefore, if the load is connected to the inverter when the inverter is started, the engine may be stalled.
In an inverter controlled generator set shown in Japanese Patent Application Laid-Open Publication No. 2001-128463, an operation of an inverter is started when it is confirmed that a predetermined time is passed and that an output voltage of a DC power source section becomes higher than a set value, after an engine is started and a microprocessor of a controller is started.
Also, in the inverter controlled generator set, in order to improve a fuel consumption rate of the engine and prevent an air pollution caused by an exhaust gas, a rotational speed of the engine is controlled so as to maintain the output voltage of the DC power source section at the set value.
As a generator driven by an engine, a magneto generator is often used. As it is widely well known, the magneto generator comprises a magneto rotor and a stator which includes a core having a magnetic pole portion opposed to a magnetic pole of the magnetic rotor and an armature coil wound around the core. An output characteristic (a characteristic of an output voltage VD versus an output current ID) of the magneto generator show a drooping characteristic, and the output voltage is rapidly decreased following an increase of the output current. The output characteristic of the magneto generator is determined by a constitution of a magnetic field and a constitution of a winding.
Now, it is assumed that the output characteristic of the DC power source section is a characteristic shown in a curve A in FIG. 5, in the case where the magneto generator is used as a generator and a rotational speed N [rpm] of the engine is constant. If the output voltage of the DC power source section required for outputting the rated voltage from the inverter is set Va, and the output current of the DC power source section when a rated load current flows is set Ia, an operating point is P when a rated load is applied. In the case where the output characteristic of the DC power source section is the characteristic shown in the curve A, when the inverter controlled generator set is put in a non-load state by disconnecting the load while the rotational speed of the engine is constant, the output voltage of the DC power source section is increased to Vo.
On the other hand, in order to output a constant rated voltage from the inverter, the DC power source section may generate the output voltage Va. Thus, a characteristic required for the DC power source section may be a characteristic outputting the constant output voltage Va between the non-load state and a rated operating state as shown in a characteristic B in FIG. 5.
As is apparent from FIG. 5, when the rotational speed of the engine is kept constant, the output voltage of the generator becomes higher than a required voltage when the load becomes lighter, which causes the engine to waste fuel. Thus, in the inverter controlled generator set, a constant voltage control is performed. In the constant voltage control, the output voltage Va of the DC power source section required for outputting the rated voltage from the inverter voltage is determined as a set voltage, and the rotational speed of the engine is controlled so that the output voltage Va of the DC power source section is kept at the set voltage in response to a change of the load of the inverter.
In the constant voltage control, the output voltage of the DC power source section is maintained at the set voltage Va in response to the change of the load as described hereinafter. As shown in FIG. 6, the rotational speed of the engine is controlled to maintain, in response to the change of the load, the output voltage of the DC power source section at the set voltage Va so as to let the rotational speed of the engine be N1 and the output characteristic of the DC power source section be a characteristic in a curve C when the inverter is in a no-load state, and to let the rotational speed of the engine be N2(>N1) and the output characteristic of the DC power source section be a characteristic in a curve D when the load of the inverter is, for example, one half of the rated load. Also, the rotational speed of the engine is controlled to maintain, in response to the change of the load, the output voltage of the DC power source section at the set voltage Va so as to let the rotational speed of the engine be N3(>N2) and the output characteristic of the DC power source section be a characteristic in a curve A when the load of the inverter is the rated load. Since the rotational speed of the engine is lowered when the load of the inverter is light or in the no-load state, such a constant voltage control not only improve the fuel consumption rate but also prevent an air pollution caused by exhaust gas by reducing an exhaust amount of CO2.
As aforementioned, in the inverter controlled generator set, when the rotational speed of the engine is controlled so as to maintain the output voltage of the DC power source section at the set voltage according to the change of the load, the fuel consumption rate of the engine can be controlled, and the air pollution caused by exhaust gas can be reduced. Therefore, as shown in the inverter controlled generator set disclosed in Japanese Patent Application Laid-Open Publication No. 2001-128463, the constant voltage control as stated above may be performed in the case where it is confirmed that the predetermined time is passed and that the output voltage of the DC power source section becomes higher than the set value, after the engine is started-and the microprocessor of the controller is started.
In the case where the control of the inverter is started when the predetermined time is passed after the CPU is started and when the output voltage of the generator becomes higher than the set value, there is no problem to perform the constant voltage control to lower the rotational speed of the engine, since almost no load is applied to the generator until the inverter is started. However, in the case where a large load is connected to the inverter when the inverter is started while the rotational speed of the engine is lowered by the constant voltage control, the output voltage of the generator is lowered significantly since the large load is applied to the generator by flowing a large load current through the generator at the moment when the inverter is started. At this time, the constant voltage control is operated in order to increase the rotational speed; however, the constant voltage control cannot increase the rotational speed enough when the load is large, which makes difficult to drive the load. In addition, the engine may be stalled if the inverter is started in a condition where the engine is in an unstable state, for example, a condition where the warm-up of the engine is not completed.